The four contested islands, seized by Russia during World War II, have gained importance since the end of the conflict, providing strategic military bases to the Kremlin, coupled with their rich mineral deposits and fishing grounds.
Russian President Vladimir Putin visited the far-eastern Kuril islands for the first time on Thursday, prompting a "strong" protest by Japan which also claims the territory.
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Russian-Japanese relations have long been dogged by tensions over the contested islands, known in Japan as the Northern Territories and seized by the Soviet Union in 1945.
The Kremlin posted a video of Putin in the Kuril Islands, with Russian media saying he visited a fish-processing plant where he was given caviar. Putin travelled to the Kurils after visiting the nearby Russian island of Sakhalin, where Russia's navy was conducting drills.
In Sakhalin, Putin held a meeting "on ensuring the security of Russia's eastern borders," state-run media outlet RIA Novosti said.
Japan, whose ties with Russia have worsened further since the full-scale invasion of Ukraine in 2022 – just as Moscow's relations with China and North Korea have warmed – objected strongly.
"The Northern Territories are an inherent part of Japan's territory both historically and under international law, and the government of Japan strongly protests this visit," said Foreign Minister Toshimitsu Motegi in a statement.
The Soviet Union took the strategically located volcanic archipelago north of Hokkaido in the final days of World War II, and has maintained a military presence there ever since.
In 2010 then-president Dmitry Medvedev became the first Russian leader to visit the disputed territory. Medvedev also went there several times subsequently as prime minister under Putin’s presidency.
The four contested isles are part of the volcanic Kuril archipelago, which runs in a broad arc from Russia's Kamchatka peninsula down to Japan's main northern island Hokkaido. The Soviet Union only declared war on Japan on August 8, 1945, after the atomic bomb was dropped on Hiroshima and six days before the end of the conflict.
The islands' names are similar in both countries -- Kunashir, Habomai, Shikotan and Iturup in Russian, while in Japanese they are known as Kunashiri, Habomai, Shikotan and Etorofu.
Their population of about 20,000 Russian citizens live in harsh climates, but the islands have mineral deposits including gold and silver, and rich fishing grounds. After the islands were occupied in 1945, Moscow deported their Japanese population of about 17,000 people.
Thereafter the islands acquired strategic importance, providing air and sea bases during the Cold War as the Soviet Union confronted the United States and its closest ally in the Pacific, Japan.
On several occasions it appeared that off-and-on talks between Tokyo and Moscow were making progress and in 1956, when diplomatic relations were established, where Soviet leader Nikita Khrushchev suggested two of the disputed islands might be returned under a peace treaty.
Although the talks resumed after the fall of the Soviet Union in 1991, no progress has been made and they remain in Russian control.